WASHINGTON—Just more than a quarter of the eight million people who signed up for health plans under the Affordable Care Act are in the prized demographic of 18 to 34 years old, falling short of the figure considered ideal to keep down policy prices.

The data, released Thursday by the Obama administration, painted a more complete picture of enrollment in the plans. They show that about 28% of people picking plans on the state and federal insurance exchanges by April 19—after most states' enrollment deadlines passed—were 18...